PGO thin film refers to Pb5Ge3O11 ferroelectric phase. Although c-axis PGO usually exhibits layered microstructure, during the deposition process it is difficult to form single phase c-axis PGO thin film having a very smooth and uniform surface. One reason is that the PGO phase is polycrystalline. However, there are a few other lead germanium oxide compounds, which are very close to the Pb5Ge3O11 phase, both in composition and formation temperature, and which are easier to form under similar conditions. If multiple phase lead germanate, having different microstructures, is formed on the surface of a bottom electrode at the same time, it is difficult to obtain a smooth and uniform c-axis PGO thin film. Several factors affect the formation of single-phase, c-axis PGO thin film, one of which is the surface condition of the bottom electrodes. The lattice constant matching is an important factor to form layered c-axis PGO thin film. The microstructure of PGO phase is hexagonal structure having lattice constants of a=10.251 Å and c=10.685 Å. For pure iridium (Ir) and platinum (Pt) metal bottom electrodes, which are face-centered-cubic (FCC) structures having lattice constants of a=3.83 Å and a=3.92 Å, respectively. Theoretically, it is relatively difficult to obtain the c-axis PGO single-phase on both electrodes. However, while this is true for a Pt substrate, c-axis PGO film may be formed relatively easily on an Ir substrate. This may be due to the thin layer of IrO2 which forms on the Ir surface in situ during the deposition and annealing process, which may assist the c-axis PGO nucleation and grain growth. IrO2 has lattice constants of a=4.498 Å, c=3.154 Å.
The orientation of the bottom electrode is also very important for the phase formation of the PGO thin film. It has been found that amorphous and polycrystalline substrates promote the formation of a smooth and uniform PGO thin film. A strong oriented substrate, having mismatched lattice constants tends to promote formation of polycrystalline ferroelectric PGO thin film having other secondary phases, wherein the film exhibits a rough surface.
FIG. 1 depicts a PGO thin film formed on a patterned substrate by MOCVD. The light area in FIG. 1a is a polished Pt substrate area, the darker areas are SiO2 substrate. Both types of substrates are polished and planarized. FIG. 1b depicts the crystalline structure of a PGO thin film formed on the Pt (left) and SiO2 (right) substrate. It is clearly seen that the PGO thin film formed on the Pt substrate is polycrystalline and exhibits a rough surface. The PGO thin film formed on the SiO2 substrate exhibits a layered single-phase structure. The PGO thin film formed on the SiO2 substrate is single-phase c-axis PGO thin film.
The thermal stability of the electrode is also important in order to form a smooth and uniform single-phase c-axis PGO thin film. It has been found that both Pt and Ir tend to form hillocks during high temperature annealing, e.g., above 500° C., which affects the nucleation and orientation of PGO thin film. An Ir composite electrode, however, is very stable during even very high temperature annealing in oxygen ambient.
The existence of oxygen in the bottom oxide electrode also plays an important role. Because both the PGO and bottom electrode are metal oxide, the favored bonding condition between the oxides at the interface can increase nucleation density help in the formation of a smooth c-axis PGO thin film.
Fengyan Zhang, Tingkai Li, Douglas J. Tweet and Sheng Teng Hsu, Phase and microstructure analysis of lead germanate thin film deposited by metalorganic chemical vapor deposition, Jpn. J. Appl. Phys. Vol. 38, pp 59–61 1999, discusses various phases of lead germanate as formed in thin films.
Fengyan Zhang, Jer-shen Maa, Sheng Teng Hsu, Shigeo Ohnish and Wendong Zhen, Studies of Ir—Ta—O as high temperature stable electrode Material and its application for ferroelectric SrBi2Ta2O9 thin film deposition, Jpn. J. Appl. Phys. Vol. 38, pp 1447–1449, 1999, describes the use of a Ta barrier layer and an Ir—Ta—O electrode.
Fengyan Zhang, Tingkai Li, Tue Nguyen, Sheng Teng Hsu, MOCVD process of ferroelectric lead germanate thin films and bottom electrode effects, Mat. Res. Soc. Symp. Proc. Vol. 541, pp 549–554, 1998, describes growth of c-axis PGO thin film.